1. Field of the Invention
The present invention relates to modems using QAM modulation techniques and in particular to a circuit for selectively eliminating phase jitter in the absence of white noise.
2. Description of the Prior Art
Phase jitter and frequency offset are two line impairments which have a detrimental effect on the incoming signal in a digital modem receiver. To prevent these impairments from causing errors in the demodulated modem signal a phase tracking loop may be utilized, as disclosed in the commonly assigned co-pending application entitled Phase Tracking Loop for Digital Modem, Ser. No. 407,451, filed on Aug. 12, 1982 now U.S. Pat. No. 4,532,640 granted on July 30, 1985, and incorporated herein by reference. The phase tracking loop was found to be very effective with respect to frequency offset and low frequency, low amplitude phase jitter while high amplitude phase jitter sometimes cannot be tracked out by the phase tracking loop.
Therefore, in co-pending, commonly assigned patent application entitled Phase Tracking Loop Impairment Monitor for Modems, Ser. No. 447,989, filed Dec. 8, 1982, now U.S. Pat. No. 4,534,036, granted on Aug. 6, 1985, and incorporated herein by reference, a device is described in which a corrected phase error signal, an uncorrected phase error and a total phase error signal is derived from the instantaneous phase error signal generated by a phase tracking loop, for monitoring purposes.
The phase tracking loop described hereinabove essentially eliminated long term tangential errors in the received signals. However, data transmission channels are also subject to error signals which do not have a long-term detrimental effect on the phase of the incoming signals. One such type of signal is white noise, which has an instantaneous radial and tangential component. The phase tracking loop normally compensates for the tangential component of the noise signal thereby degrading the performance of the receiver.